In the operation of smelters and furnaces for recovery, refining and upgrading of ores, minerals, and metals large amounts of impurities including inorganic dusts, dirt, ash, and slag are carried by the flames and exhaust gases to the outlet of the furnaces. Some of these particulate impurities tend to settle on the throat or uptakes while other particles tend to settle and collect throughout the outlet system. The problems are common to most smelting and refining operations, however the following discussion will be directed to copper processing.
In operation of copper reverberatory furnaces, the ore or concentrate charged to the furnace contains about 30%Cu, with the remainder being iron, silica, and other non-copper materials. The charge is of small particle size because it is usually a concentrate from a flotation system. Reverb furnaces depend on oil or gas to provide heat to melt the charge into a matte (copper) and slag. Matte is a lower liquid, slag a floating molten material going to waste. Under some conditions there may be an intermediate layer of magnetite separating matte and slag, or there may be a buildup of magnetite on the bottom of the furnace. The charge is dumped onto the slag layer, either from the hot firing end of the furnace, or from the two sides of the furnace.
In both feed systems dusting during charging and the gas velocity of the flame will result in mineral dusts being carried along by gases through the reverb, out of the reverb through an exhaust outlet called an uptake, and then into a waste heat boiler. Some of the mineral particles will settle out in the uptake where the particles sinter or fuse and accumulate into a mass large enough to cause operational difficulty -- sometimes reverb shutdown.
Removal of this build-up caused by sintering and fusion is necessary, expensive, and difficult. Past techniques include melting by additional heat, use of a shot gun slug to pound off build-up, jack hammering away chunks of build-up, adding a flux (soda ash -- ferrosilicon) by shovel, air gun, or hand throwing, or an oxygen lance.
Build-up removal has two distinct problems:
1.thinning out refractory walls to a dangerous degree because of poor control during removal of undesired deposits, and PA1 2. incomplete removal can lead to uptake build-up to the point where restricted flow is critical. PA1 b. Hydrous powdered silicates also will dissolve in their water of hydration at temperatures of about 60.degree. to 90.degree. C to give viscous adhesive particles. These powders normally contain 10 to 30% water by weight and would have an SiO.sub.2 /M.sub.2 O mole ratio of 1.1 to 3.9/1.0, PA1 c. Anhydrous soluble alkali glasses will soften at temperatures of 1000.degree. to 1300.degree. C to form sticky masses. When these glasses are premoistened or dampened they will become sticky at 60.degree. to 90.degree. C and adhere to the surfaces of the equipment interior when they come in contact therewith. Sodium silicate glasses having an SiO.sub.2 to M.sub.2 O mole ratio of less than 2.2 to 1.0 and potassium silicate glasses having an SiO.sub.2 to M.sub.2 O mole ratio of less than 3.92 to 1 are suitable for use in this process, and PA1 d. Crystalline hydrates of sodium sesquisilicates would likewise be utilized in this process as the sesquisilicate will form an adherent mass in its own water of hydration.
A removal technique should be precise enough to remove buildup but not attack the thin sections of the uptake, and also should not aggravate solids carry-over into the waste heat boiler. The flux shoud not be so aggressive as to erode or attack refractory sections below the build-up, or any other section of the equipment encountered as the fluxed molten build-up drains into the slag area.
It must be emphasized that while most process equipment has build-up problems, these problems vary extremely in terms of severity, ease of handling, composition, location of build-up, rate of formation of build-up, and the physical geometry or design in terms of ease of getting at or removing a build-up. The build-up is related to the amount of ore fed in, its mesh size and composition, whether it has been roasted or pretreated in a fluid bed reactor, the fuel used in the reverb (gas is easier on a reverb than oil), the degree of overloading the reverb for increased production, use of auxiliary oxygen for heating, end charging versus side charging, operating temperature of the furnace, the type of refractory used (basic vs. silica), whether the reverb is operated under oxidizing or reducing conditions, and the general air tightness of the reverb.
Removal of build-up and the technique used has always been a function of quality of labor available and its cost, operating philosophy, equipment and materials available, geometry or design of the furnace and the uptake, type of refractory, composition of build-up, temperature in the uptake, and the rate of formation of build-up. Each reverb and its build-up has had to be treated and solved as a unique situation. There was no simple, pat combination of materials, equipment, or operating technique universally applicable to all conditions.
Any deposition of solids, particularly the inorganic impurities of the metal processing, out of the melting area of the smelter or furnace is objectionable for various reasons. The materials tend to restrict flow of exhaust gases by reducing the cross-sectional area of the exhaust vent. The material can cause poor heat transfer to the refractory surface. Build-up of the material can cause damage to the refractory itself. The excessive build-up can actually be the cause for shutdown of the furnace in order to remove the build-up. Extensive labor and/or hazardous conditions may be required for removal of the build-up. Build-up can damage boiler tubes and it can reduce overall thermal and melting efficiency of the equipment.
The build-up material found in these processes generally contains high ratios of inorganic dust, and it has been the prior art practice to introduce alkali-containing fluxing agents which tend to form a glass with the build-up material that will run back into the melt of the equipment as slag. Some of the more common and recognized fluxing materials are soda ash, ferrosilicon, calcium carbide, lime, iron pyrites and silica. Soda ash is probably the most common as it is a very low cost fluxing agent. It has poor particle size, shape and density; it blows away very easily, and it does not have any tendency to stick to the surface of the build-up. Its effectiveness is based upon the ability of a small percent of the soda ash applied to react with the build-up while in contact with it. While it is very cheap, most of the soda ash is not utilized but is carried away with the exhaust gases, presenting new problems in an air pollution conscious environment. The soda ash also tends to react with or attack the refractories.
Ferrosilicon is a highly effective, high density powder which is rather costly as a fluxing agent. It will react with magnetite in the build-up to reduce iron in producing low melting slag. In some instances it can be too reactive and go through refractory brick easily.
Calcium carbide has a fluxing mechanism and is not completely understood; however it is believed that the carbide is a reducing agent which probably reacts with the metal components in the build-up. Calcium carbide does create a run-off of the build-up material which is rather thin and tends to boil violently.
Lime is generaly used in combination with some of the other fluxing agents, such as soda ash.
The iron pyrites have generally been suggested as suitable substitutes for ferrosilicon, being at a lower price, with some loss in effectiveness.